The present invention relates to a cermet used for cutting tools, milling tools, etc., particularly to a TiCN-based cermet having excellent crack resistance and wear resistance.
Cermets are typical materials for cutting tools like cemented carbides, though the former is slightly poorer in toughness than the latter. Thus, a lot of attempts have been made so far to improve the toughness of cermets. The most effective means may be the addition of TiN or TiCN. Their addition contributes to improvement in toughness, because the microstructures of cermets are made finer with TiN or TiCN. Investigation is presently conducted on further increase in the N content, fine pulverization of material powder and sintering techniques to improve the toughness of cermets. Thus, ultra-fine grain cermets similar to ultra-fine grain cemented carbides are now available.
In-depth research has also been conducted on the microstructures of cermets. For instance, Japanese Patent Laid-Open No. 11-131170 proposes an excellent cermet tool obtained by optimally controlling the shape of TiCN particles in its structure. Japanese Patent Laid-Open No. 9-300108 proposes a cermet tool excellent in wear resistance, which is obtained by causing TiWMCN, wherein M is at least one of Zr, V, Nb and Ta, to surround TiCN particles in the process of sintering.
Though a lot of research has conventionally been conducted on improvement in the toughness of cermets, drastic progress has not been achieved yet. Apart from the problem of toughness, cermets are subjected to extremely rapid notch wear than cemented carbides, and it is sometimes observed that the notch wear restricts the life of tools. Particularly in the case of cutting materials having relatively high tensile strength such as hot-working tool steel, such phenomenon is extreme.
The causes of generating notch wear in cermet tools are considered in many ways such as oxidation wear, damage due to rapid change of thermal gradient, biting by chips remaining between a tool and a work, etc. Though contribution of each cause has been verified to some extent, the inventors have considered that they are not decisive causes. If the mechanism of generating notch wear of cermet tools were found so that the notch wear can be prevented, and if the toughness of cermets were further improved, the cermets would be provided with further improved properties suitable for tools.
Thus, an object of the present invention is to provide a cermet with improved notch wear resistance and toughness.
Thus, the TiCN-based cermet according to the present invention comprises 5-25 weight % of a binder phase mainly composed of Co and/or Ni, the balance being substantially a hard phase and inevitable impurities, the hard phase being mainly composed of carbide, nitride and/or carbonitride and containing at least Ti and W, the cermet having a cross-section microstructure in which the number of Ti-rich particles having an area of 0.02 xcexcm2 or more is 1000 or less per a unit area of 1000 xcexcm2.
In a preferred embodiment of the present invention, the TiCN-based cermet has a crack resistance of 60 kg/mm or more. The TiCN-based cermet preferably has a cross-section microstructure in which the number of Ti-rich particles having an area of 0.02-0.4 xcexcm2 is ⅔ or more of the total number of Ti-rich particles having an area of 0.02 xcexcm2 or more.
The TiCN-based cermet is preferably coated with a hard material.